Chemotaxis under flow disorder shapes microbial dispersion in porous media

Bacteria live in heterogeneous environments, so it is important to investigate their behaviour in porous media. Here the authors show that flow disorder enhances the effect of chemical gradients in micropockets in a porous medium, which then aid the transport of bacteria. Natural soils are host to a high density(1)and diversity(2)of microorganisms, and even deep-earth porous rocks provide a habitat for active microbial communities(3). In these environments, microbial transport by disordered flows is relevant for a broad range of natural and engineered processes, from biochemical cycling to remineralization and bioremediation(4-7). Yet, how bacteria are transported and distributed in the subsurface as a result of the disordered flow and the associated chemical gradients characteristic of porous media has remained poorly understood, in part because studies have so far focused on steady, macroscale chemical gradients(8-10). Here, we use a microfluidic model system that captures flow disorder and chemical gradients at the pore scale to quantify the transport and dispersion of the soil-dwelling bacteriumBacillus subtilisin porous media. We observe that chemotaxis strongly modulates the persistence of bacteria in low-flow regions of the pore space, resulting in a 100% increase in their dispersion coefficient. This effect stems directly from the strong pore-scale gradients created by flow disorder and demonstrates that the microscale interplay between bacterial behaviour and pore-scale disorder can impact the macroscale dynamics of biota in the subsurface.